Laminated security window system

ABSTRACT

A security window system having a transparent structure having high resistance to penetration is described. A transparent conductive layer is provided over most of the area of the window and the resistance of the layer is monitored for sensing penetration. Preferably the layer is subdivided into a number of conductive regions for substantially increasing the sensitivity of the system to minor interruptions in the layer. Temperature and stress effects can be minimized by connecting different conductive areas of the layer as arms of a resistance bridge. An alarm may be sounded when a small steady state change in resistance is sensed or when a rapid change in resistance is sensed.

Unit ed States/Patent 191 Nelson et al.

'LAMINA'l'ED SECURITY wmnow SYSTEM Inventors: Roger E. Nelson, Northridgfi; Clyde L. Lucky, Santa Susana, both of Calif.

The Sierracin Corporation, Sylmar, Calif.

Filed: Nov.16, 1972 Assignee:

,u.s. c1..,.., 340/274, 52/616, 109/21,

. 1 161/404, 340/285 1m.c1. G08b13/04 "Field of Search..'..'340/274,'285 109/21,v 10,

{References Cited UNITED STATES PATENTS 12/1958 -Danfo1'd.... 52/1711/1960 Boicey 340/274 4/1965 Ryan et al 161/192 1111 3,825,920 July 23,1974 7/1971 l-lametal. .Q 340/285 6/1972 1.10611, .lr. 109 495 PrimaryExaminer-John W. Caldwell Assistant Examiner-Glen R. Swarm, IIIAttorney, Agent, or Firm-Christie, Parker & Hale [57] ABSTRACT v Asecurity window system having a transparent structure having highresistance to penetration is described. A transparent conductive layeris provided over most of the area of the window and the resistance ofthe layer is monitored for sensing penetration. Preferably the layer issubdivided into a number of conductive regions for substantiallyincreasing the" sensitivity of the system to minor interruptions. in thelayer. Temperature and stress 'effects'can be minimised byconnecting'different conductive areas of the layer as arms of aresistance bridge. An alarm may be sounded when awsmall steady/statechange in resistance is sensed or when a rapid change in resistance issensed.

l6'Clalms, 13 Drawing Figures PATimEnJmzawn sum 2 or 4 AME/l4PATENTEDJULZBIHH SHEET 3 BF 4 n L MW 7 I! vvv V b/v vvv. 1

QR mm window that is relatively impenetratable. Such windows maybe usedin prisons, hospitals, museums, z oos,'computer rooms, laboratories, orin store fronts where'theft or vandalism maybe a problem. They areuseful any placewhere maximum natural lighting, 'visual access andphysical security are requisite. Jewelry counters and'laboratoryhoodsare other suitable locations. Bars can be added adjacent ordinary glass;however, this is in many cases undesirable fora variety of reasons.

Thus, in a prison or hospital or similar institutions, bars may haveasignificantly undesirable effect on persons withintheins'titutionrBars' detract from'the pleasure of visitors'to 2005 or.museums.v In stores and the like where protection is desired againstentry, the-presence Of'bars, is highly undesirable because of theadverse effecton potential customers-Collapsible window grates arelittle better. I I 1 Windows that are highly resistant to'penetrationcan be formed with thicklayers of glass or preferably with laminatedglass sandwiches which may include layers of tough, impact resistantplastics, such as the polycarbonateplastics. 'Tempered glass isdesirable in some sitnations in case of breakage. Thus, for example, insome institutions persons may deliberately break windows to obtainslivers of glass to use as weapons or toingest in a suicidal act.Tempered glass is desirable for such situa tions. since it does notshatter like ordinary glass but breaks into relatively of all sharpedges. 3 a

In addition to resistance to penetration it is often highly desirable toprovide sensing of efforts to penetrate so that an alarm can be soundedlocally or at some remote station. Thus, for example, penetration of aprison window indicateseither an escape attempt or an effort to conveycontraband. Sensors in the individual prison windows can be monitored ina central location for detection of such unlawful activities. Similarly,in stores or the like, breakage of a window commonly precedes a burglaryattempt. For this reason, burglar alarm systems commonly include means.forsensing breakage of the window. v

A very common technique for sensing breakage of a window is to adhere aconductive tape such as thin aluminum or lead foil directly to the glassaround the periphery of the window. Such strips are unsightly and arepreferably avoided, particularly in store windows and the like where anattractive appearance is highlydesirable'. Omission of the obvious alarmstrips may also be small fragments substantially free ative untilsomeone gets to the window andbridges a break in the tape. There is noway of resetting such an alarm from a remote location.

It is therefore desirable to provide a security'window system having'analarm built into it which is sensitive to attempts to penetrate thewindow and which can be reset from a remotelocation. It is desirable tohave a signal from the window that is related to the degree ofpenetration, which in this context'can be considered to be an analogchange as compared with the binary change that occurs upon completeinterruptionv of an electrical path. Preferably such a security windowis substantially free of apparent visual indications of the presence ofthe alarm. For most uses the window is preferably resistant topenetration with the impact re-' sistance'of polycarbonate and theresistance to sawing that is characteristic of glass. i A

BRIEF SUMMARY or THE INVENTION" There is, therefore, provided inpractice of this invention according to a presently preferred embodimenta security window system having a transparentielectrijcally conductivelayer over most of the area of the window. Means are provided for makingelectrical contact with edges of the conductive layer so that an analogchange in resistance can be detected. An output signal can then be givenin response to a change in resistance 9 in excess of a predeterminedmagnitude.

DRAWINGS:

FIG. 4 is a face view of another embodiment of security window; a k

FIG. 5 is'a block diagram of a sensing circuit for the window of FIG. 4;I

FIG. 6 is a face view of another embodiment of security window;

FIG. 7 is a face view of still another embodiment of security window; 1

FIG. 8 is a partially cut-away section of the window of FIG. 7;

FIG. 9 is a fragmentary cross section of another embodiment of securitywindow;

FIG. 10 is'a fragmentary cross section of another embodiment of securitywindow;

FIG. 11 is a schematic diagram of another penetrationsensing system; i p

FIG. 12 is a schematic diagram of another resistance sensing technique;and

FIG. 13 illustrates another embodiment of security window and aschematic circuit connected thereto.

DESCRIPTION FIG. 1 is a face view of a security window and FIG. 2 is afragmentary cross section showing the laminated layers thereof. In faceview the security window apof a sensing circuit for the windowof FIG. 1;

pears much like an ordinary transparent window except that it may appearsomewhat tinted or have slightly less light transmission than anordinary clear glass window. In addition, very narrow isolation lines20, described in greater detail hereinafter, may be seen in the face ofthe window. Ordinarily these lines are'very minute and not noticeableexcept on close examination. Metallic bus bars 21 are imbedded alongopposite side edges of the security window. A short tab 22 from each ofthe bus bars typically extends beyond the edge of the window for makingelectrical contact. The bus bars are preferably'imbedded corrugatedcopper strips as described in U.S. Pat. No. 3,612,745. Other suitablebus bar arrangements vwill be apparent to one skilled in the art, suchas theexternal bus bars of U.S. Pat. No. 3,529,074. Typically, when thesecurity-window'is used it is mounted in a frame'so that the edgeportions are all hiddenand the bus bars 21 are thereby hidden by theopaque frame. This is desirable so that the appearance of the securitywindow essentially matches the appearance of an ordinary glass window.

The various laminations forming the cross section of the securitywindow-are illustrated in the fragmentary view of FIG. 2. A sheet oftempered glass 23 forms one face of the window. As will beapparenthereinafter it ispreferred that this face be the one from whichpenetration is most likely to occur. Typically the tempered glasslayeris about one quarter inch thick. A transparent resilient plasticinterlayer 24 is securely bonded to the glass'sheet 23. This interlayeris thesame as that typically employed in laminated automobile glass, forexample. A layer 0.030 inch thick of polyvinyl butyral makes a suitableinterlayer that is conveniently bonded to the other layers of thelaminated window by conventional heat and pressure laminatingtechniques.

A carrier film 25 having a metal layer 26 on one face thereof isbondedto the plastic interlayer 24. It is relatively unimportant whichface of the carrier film has the metallic layerthereon. The carrier filmis, for example, a film of polyethylene terephthalate about 0.005 inchthick. The metal layer 26 is an extremely thin layer of a metal such asnickel, gold, silver, aluminum, copper or the like which can be vacuummetallized onto the carrier film. Such vacuum deposition of thin metalfilms is a conventional process widely used for preparing electricallyheatable windows. The metal coating is deposited in a sufficiently thinlayer that it is transparent and'absorbs relatively minor amounts ofincident,

light so that the overall transmission characteristics of the window arenot substantially diminished. The metal layer is sufficiently continuousto have a substantial electrical conductivity.

By employing a thin carrier film the metal layer may be vacuum depositedon the carrier film by a continuous process whereby large sheets ofcarrier film are coated and subsequently cut to a desired size.Relaatively uniform resistance throughout the metal layer can beachiever by such a vacuum metallizing .treatment. The conductive layer26 extends over most of the only requirement is that the conductivelayer extend near enough the edges of the security window to make goodelectrical contact with the bus bars 21 (FIG. 1)

adjacent the edges of the sheet and extend over most of the area of thewindow where penetration may be likely to occur. Ths bus bars areimbedded in the laminate between the interlayer 24 and the carrier film25 so as to be in electrical contact with the metal film.

Another interlayer 27 is bonded to the opposite side of the carrier film25 from the first interlayer 24. These interlayers are substantiallyidentical. An impact resistant plastic ply 28 is bonded to the secondinterlayer 27. A variety of transparent impact resistant plastics aresuitable for use in such a security window. Methyl methacrylate resinmay be employed, for example. it is preferred, however, to employ apolycarbonate resin for the plastic ply. This material is commerciallyavailable under the trademark Lexan from General Electric and under thetrademark Merlon from Mobay Chemical Company. The polycarbonate sheet isextremely impact resistant and has a high transparency. Thus, even ifthe tempered glass layer 23 is broken the polycarbonate layer 28normally resists impact penetration. Such a polycarbonate sheet, forexample, may be about one quarter inch thick.

Another polyvinyl butyral layer 29 is bonded to the other side of theimpact resistant ply 28. This interlayer is substantially identical tothe first two. Finally a second sheet 30 of tempered glass is bonded tothe third interlayer 29 and forms the other face of the laminatedsecurity window. This second layer of tempered glass is also about onequarter inch. The glass and plastic layers have differing coefficientsof thermal expansion and when temperature cycling is expected it isdesirable to provide stress relief around the periphery of the window. Asuitable edge separator technique is provided in copending U.S. PatentApplication Ser. No. l 1 1,993 by Jan B. Olson, entitled interlayerStress Reduction in Laminated Transparencies and assigned to SierracinCorporation, assignee of this application. The polycarbonate layer maybe subject to attack by plasticizers in thepolyvinyl butyral layer andit is usually desirable to employ a polycarbonate sheet with a barrierlayer on its faces. Such a coated polycarbonate material is availablefrom General Electric under their trade designation MR4000. Any of avariety of conventional transparent melamine, phenoxy or urethane resinsform suitable barrier layers.

The security window illustrated in FIG. 1 is highly resistant topenetration since the tempered glass has substantial impact resistance.Even if the glass layer is broken by scratching or sharp impact thepolycarbonate layer has much higher strength and ordinarily hassufficient impact resistance to prevent penetration. Tempered glassbreaks into alarge number of relatively small particles and theseparticles remain bonded to the interlayer. The presence of such a massof glass fragments on the surface of the window does a great deal toinhibit sawing or other cutting of the polycarbonate plastic.

There are substantial advantages to having a security window formed witha glass face layer and a polycarbonate plastic layer laminated togetherin combination with an alarmas herein described. It will be noted thatthe carrier film where the conductive layer forming the analog sensor ofthe alarm circuit is located is separated from the glass andpolycarbonate layers by a relatively soft and flexible polyvinyl butyralinterlayer. lf the frangible glass layer is broken, as by a sharplocalized blow or a deep scratch which may trigger fracture of temperedglass, the glass fragments are largely held in place by adhesion to theinterlayer. The cracks from the glass seldom penetrate the resilientinterlayer and hence-do not interrupt the thin metal film. Thus the merefact that the glassis broken does not necessarily trigger an alarm. Thesame is not true of a system wherein the alarm sensor comprises aleadtape around the periphery of the window or an electricallyconductive filmapplied directly to the glass. In such a system a crackpropagating to the edge of the windownormally results in breaking of thelead tape or conductive film and triggering of an alarm. v

I The resistance sensor extending over most of the area ofthefwindow'therefore serves to detect penetration of the window. When ahole is madein the window of a sufficient size to interrupt a portion ofthe conductive layer, the alarm will be triggered. Mere cracking of thewindow or surface damage will not ordinarily trigger the alarm.Detectioniof penetration .is what is; sought and this is provided by thecomposite laminated win- I dowwith aconductive layer embedded therein.

Referring again to FIG. 1, the conductive layer is in electrical contactwith the bus bars 21' along opposite edges of the window. A resistiveconnection is thereby provided betweenthe, two bus bars. The isolationlines are actually extremely fine scribelines made in the face of thecarrier-film on which the conductive layer is deposited.Since thisconductive layer is extremely thin a scribe line that is nearlyinvisible to the naked eye is sufficientforinterrupting the electricalcontinuity of layer in a direction parallel to the bus bars, a straightline out was made through the conductive layer in a direction parallelto the bus bars. A cut extending more than 20 percent of the way betweenthe side edges of the conductivelayer increased the resistance less than1.1 percent. A circular interruption in the conductive layer having adiameter of about 17 percent of the width of the conductive layer causedan increase in-re sistance of only about 2.6 percent.

When isolation lines are scribed through the conductive layer in adirection extending between the bus bars the conductive film is dividedinto a plurality of'conductiveareas that are electrically inparallel'with each other. Then when a sufficient cut is made parallel tothe bus bars to completely sever one of such parallel conductive areas ajump in resistance occurs. Thus, for example, a conductive layer wassubdivided into six conductive areas by five scribed isolation lines.Asa straight line out proceeded across one of the parallel conductiveareas a nominal gradual change in resis-' tance occurred. When one ofthe six parallel conductive areas was completely severed betweenadjacent isolation lines, an increase in resistance of about 20 thefilm. A scribe-line can be made with a shallow sharp groove that extendsinto the carrier film a tiny distance, but not even this is needed.The'metal layer isso thin out marringfthe carrier film. I

If an effort is made topenetrate the security window the electricallyconductive layer must also be penetrated. Any interruptionof theconductive layer having that almost any abrasion is enough to interruptit withn a component in a direction parallel to the bus bars will causean increase in the resistance of the conductive layer. As pointed outhereinafter the-resistance between the two bus bars 21 can bemonitored-and any significant change in resistance employed fortriggering an alarm. Any such sensing system has a predeterminedsensitivity. If thesensitivity threshold for triggering an alarm istoosmall, a significant number of false alarms may be sounded. On theother hand if the thresholdof sensitivity for triggering the alarmis toohigh, a rather large penetration of the window may occur before an alarmis sounded. It has been found that a sensitivity threshold in the areaof about one to two percent change in resistance is suitable fortriggering an alarm,

althoughfihigher or lower changesare also suitable thresholds.

If a security window is made with a continuous conductive layer overmost of the area of the window without any electricalisolation linessubdividing it into a plurality of conductive areas, the change inresistance as a function of the magnitude of the interruption of theconductive, layer may; be unduly low. When the entire window between thetwobus bars constitutes a continuous conductive layer, each point oneach bus bar is in direct electrical contact with every point on theother bus bar. "Current flow between the two bus bars can thereforeoccur over a substantial area and destruction of a minor portion of theconductive layer may have a relatively minor affect on the totalresistance. Thus, for example, in one test wherein the distancebetweenthe bus bars was 1.67 times the width of the conductive percentwas observed. Since a similar length cut in a film without isolationlines would produce a resistance changeof less than about 1 percent thevalue of the parallel conductive areas can be readily seen. In addi-.

tion to increasing the sensitivity of the security window torelativelysmall penetration, the sensitivity of the circuit fordetecting a change in resistance can be readily correlated with theresistance change thatmay occur when one conductive area of a selectedwidth is sev v ered.

each 7% inches wide. Addition of only two more isolation lines cuts thewidth of each areato only 5: inches for very high sensitivity: topenetration. If. desired a large number of electrical isolation linescan be extended between the bus barsso that. the conductive layer isdivided into a number "of .narrow parallel conductors..A penetration ofthe window interrupts a number of such narrow conductors and theresistance change is the usual change due to deleting some of theresistors in aparallel array of resistors. n

The arrayof resistors in electrical parallel is considered to extendover most of the area of the window since penetration at any point willinterrupt one or more resistors. This may be true even when theresistors become narrower than the electrical isolation lines betweenthem. Thus, if desired, one could form narrow strips of conductivematerial on the carrier film with clear areas between the stripsand havea structure differing only in scale from the arrangement illustrated inFIG. 1, for example.

FIG. 3 is a schematic illustration of a system for detecting penetrationof the security window. Very broadly the penetration of the conductivelayer causes an analog change in the resistance of .the window which isa function of the extent of penetration, and the magnitude of thischange may be used for triggering an alarm. The resistance of theconductive layer 26 is represented by the resistor 26' in the schematicillustration of FIG. 3. This resistance is connected to the de- Itectinjg circuit by electrical leads 33 including the window bus barsand whatever additional leads may be desired for conveying signals to aremote location. The thin film .resistor 26 is connected in a bridgewith a resistor 34 as an adjacent arm of the bridge. A power supply36applies anelectrical signal to the resistances 26 and 34.

'The electrical signal is also applied to a fixed resistor 37 and avariable resistor 38 connected in series with a tapped resistor orpotentiometer 39. These additional resistors 37, 38 and 39 form theother two arms of a 36 can be either a voltage or current arrangement.Similarly the power supply can be either 'AC'or DC as may be desired ina particular application. Similarly the signal applied by the bridge tothe amplifier 41 can be either a differential signal, that is, withneither bridge tap grounded or connected to a circuit common,'or it maybe a single ended signal with either of the bridge connections groundedor connected to a circuit common. Many variations in'thebridgeexcitation and unbalance detection will be apparent to one skilled inthe art.

I The output of the amplifier 41 is applied to a conventional thresholddetector 42 which senses when the null balance of the bridge is outsideof a predetermined limit. A wide variety of threshold detectors may besuitable, depending on the signal selected from the amplifier in aparticular embodiment. When the threshold detector notes that the bridgeis out of balance beyond the preset limit an alarm 43 is triggered. Anydesired alarm may be used such as a bell, klaxon, light or the like.Thealarm can be adjacent the window or remotely located. One can evendispense with the threshold detector and apply the amplifier outputdirectly to an audio alarm, such as, for example, a loudspeaker. Whenthe-sound of the loudspeaker reaches some arbitrary level as noted by anindividual in the vicinity this can also serve as an alarm.

Once an alarm has sounded, and it has beendetermined that the signalis'erroneous or one decides to presently ignore the unbalance, thebridge can be read justed by means of the resistors 38 and 39 to bringit back into balance. This is quite feasible since the signal outputfrom the bridge is analog. A change in the resistance of the conductivelayer modifies the electrical signal in an analog manner. The alarmsystem can I therefore be reset by-rebalancing the bridge, all'of whichcan be done from a remote location if desired.

I Such is infeasible in a window fitted with conventional lead tapes orwith a conductive layer directly on the glass since rupture of the tapeor layer on glass is a binary output and the circuit cannot be restoredwithout access to the window and repair of the tape.

Some changes in the resistance of the conductive layer-may occurgradually even when penetration of the window is not attempted,-thus forexample, temperature changes in the window may cause resistance changesin theconductive layer of a sufficient magnitude to unbalance thebridge. One can therefore provide an automatic balance reset 44 whichsenses an unbalance and brings the bridge back to null by adjusting thepotentiometer 39. The fact of resetting of the bridge or the magntidueof the resetting may be recorded with a conventional recorder 45. Thecumulative change in resistance recorded by the recorder 45 could beused to trigger an alarm if desired. It will be apparent, of course,that the balance reset 44 may be operated by the amplified null balancesignal from the amplifier 41 so as to operate in a more analog fashionand accommodate slow drifts in the resistance balance of the bridge.Similarly, if desired the balance reset or bridge adjustment may simplycontrol operation of the amplifier 41 to'remain below the thresholdsignal. Electrical balance of the amplifier can substitute for actualbridge resistance balancing for alarm actuation as well. The automaticreset 44 and recorder 45 are not essential to the'functioning of thesystem.

It will also be apparent that a high degree of sophistication may alsobe incorporated in the balance detection system so that, for example, asingle transient of resistance can be ignored and a more permanentchange employed for, triggering the alarm. Means may also be providedfor triggering the alarm in case the bridge leads are shorted or cut, orif the power is cut off, or if any of a variety of techniques areemployed for circumventing the alarm system.

As mentioned above, the resistance of the conductive layer in the windowmay vary with temperature and cause an unbalance of the bridge. Althoughthis can be readily accounted for with an automatic balance resettingsystem, it is also quite easy to simply compensate for the temperaturechange by making the fixed resistor 34 in the adjacent arm of the bridgeto the resistor 26 also be a conductive layer in'a window. If thetemperature pattern in the two resistors-is similar, any changes inresistance will be equivalent and the balance of the bridgewill not beupset. The second conductive layer in a window may be in a separatewindow located in a position subject to similar temperature conditionsor it may simply be another portion of the same window in whichthe layerresistor 26' is located.

FIG. 13 illustrates in face view another embodiment of security windowhaving a conductive layer extending over most of the area of the window.In this embodiment, there is a bus bar 102 extending along one side edgeof the window for making electrical contact with one entire edge of theconductive layer in the window. An electrical isolation line 103 extendsacross the window transverse to the bus bar 102 and divides theconductive area of the window into two conductive regions 104 and 106. Abus bar 107 extends part way along the side edge of the window oppositefrom the full length bus bar 102 and makes electrical-contact with theconductive layer of the first-region 104. A second similar bus bar 108extends the balance of the way across the window and makes electricalcontact with the conductive layer in the second conductive region 106.Each of the bus bars extends beyond the edge of the window formakingelectrical contact with an external circuit.

FIG. 13 also illustrates schematically a typical external circuitconnected to the bus bars of the window. A power supply 109 is connectedto the two similar bus bars 107 and 108 Resistors 111 and 112 are alsoconnected to the power supply. A first tap 113 is connected between theresistors 111 and 112 and a second tap 1 14 iszconnected to formed bythe resistors 9 the bus-bar 10 2,that makes electrical; contact withboth conductive regions 104 and -l06."The taps 1'13 and 114 may beconnected to any conventional null balance detection circuitry asdesired for-triggering analarr'n in response to unbalance of resistance.It' will be noted'that the window and external circuit illustrated inFIG. 13 are connected as a conventional bridge with the two conductiveregions of the windowas adjacent arms of the bridge. Either or botherthe resistors 1 11 or 112 can be variable for balancing the bridge, orbalancing can be achieved in the additional circuits (not shown) towhich the window may beconnectedj i t f- The two conductive regions ofthe wind'owwill both be subjected to similar temperature conditions andany changes in resistance in'the two regions willbe similar. Being inadjacent bridge arms, the resistance drift due to, temperature changebalances out and no bridge unbalance results. It will be apparent thatif desired the conductive layerin each of the conductive regions 104 and106'can be subdivided by isolation lines extending between the bus-barsto anydesired extent'forenhancing sensitivity of the; window topenetration. j FIG. 4 illustrates in face view another embodiment ofsecurity window having an electrically conductive layer over most of thearea of the window. In this embodiment, the conductive layer issubdivided into four conductivearea's46, 47, '48 and 49 by isolationlines 50. A v

sistance will be noted. As mentioned above, the conductive layerwithinthe security window changes resistance somewhat with changingtemperature. Similarly, stresses on the conductive layer which maybegenerated by bending of the window, for example, may changeresistance. When the conductive areas are interconnected as arms of abridge such thermal-or stress changes in resistance do not cause falsealarms; It will be apparent to one skilled in the art that if desiredmore than one window may be interconnected as arms of a resistancebridge and the conductive, areas may sufficiently balance to compensatefor thermal changes and the like.-This is generally less desirablesince-the thermal changes or changes in stress between two windows isusually of much greater magnitude than similar changes within twodifferent areas of the same window.

It will also be apparent-that the resistance change detector 58 shouldbeadjustable for resetting the alarm connected in series. Thus, asillustrated inthis embodi-,

ment, a plurality of isolation lines 61 extend between opposite edges ofthe security window and subdivide first bus bar Sl isin electricalcontactalong-an edge-of the firstconductive area 46. A similar short busbar 52 is in electrical contact with anedge of the other outsideconductive area 49. A third bus bar 53 is in electrical contact 'withthe edges of both of the remaining two conductive areas 47 and 4Bspanning one of the isolation lines 50.,Along the. opposite edge of thesecurity window from the first three bus barsis a fourth bus bar 54inelectrical contact with the edges of the two con- ,ductive areas 46 and47.; Another bus bar 55-on this same manner'as the window of FIG. 1 forenhanced sensitivity.

FIG. 5 illustrates schematically the interconnection .-of the conductiveareascThe bus bars and'conductive areas of FIG. 4 are representedschematically with the same reference numerals bearing a prime in FIG.5.

The two bus bars 51 and 52 are externally interconnected ata point 5152'. This same piont is connected to asuitable power supply 57 which isinturn connected to .the center bus bar 53 "at apoint 53 in theschematic illustration of FIG. ,5. Electricalconne'ction is'made to theopposite bus bars54 and 55 at the points 54" and 55 leadingto a detector58 of changes inthe electrical-resistance. The detector 58 can beconnected for triggering an alarm 59 when a predetermined liminalchangein electrical resistance occurs in the bridge (conductive areas) 46',47',

48', and 49'. a a t Since the four resistors or; conductive areas areconnected as the four arms of a resistance bridge, any resistancechanges occurring in all of the conductive areas will not unbalance thebridge and no net change in rethe area into a plurality of conductiveareas'62. Electrical contact is made along a side edge of one of theoutside conductive areas by a bus bar 63; Atab 64permits electricalconnection .of this has bar to an external circuit. At the opposite endof the'firsticonductive area from the bus bar 63 is a second bus bar 65which makes electricalcontact along the side edge of the firstconductive area and also along the sideedge of the second conductivearea adjacent the first. That is, the second bus" bar spans theisolation line 61 between the two adjacent conductive areas. Another busbar 66 electri cally connects the opposite side edge of the secondconductive area with the'side edge of the third conducti-ve area. Theseadditional bus bars 65 and 66 do not have an external tab for electricalconnection to circuits outside the window.

A similar series of additional bus bars connect adjacent conductiveareas clear across the window. In the final conductive area'a bus bar 68makes electrical connection to both the edge of the conductive'area anda tab 69 permitting electrical connection to an external circuit. Thus,all of the conductive areas within the window are electrically connectedtogether in series. Clearly a penetration that extends through the vfullextent of one of theconductive areas will interrupt the continuouscircuit and provide a substantially infinite increasein resistance.'Suchan electrical connection is 1 not resettable from a remote location.

If desired a tab 70 may be provided on a centralbus bar for. makingcontact to "an external circuit thereby permitting half of theconductive areas to be in one arm of a bridge and the other half inanother arm of a bridge for temperature and stress compensation. Such aseries connected security window is very sensitive to smallpenetrations. The same effect can be obtained without the largenumber ofbus bars by simply ending alternate isolation lines a substantialdistance from each of the opposite edges of the window respectively. Apattern of isolation lines for a series-parallel connection ofconductive areas can also be used.

[As mentioned hereinabove the security window is highly sensitive topenetrations that have a component extending in a direction parallel tothe bus bars. If the interruption in the conductive layer is primarilyin a direction between the bus bars, that is, for example, parallel tothe isolation lines, little if any change in resistance is observed.Thus, for example, if the object of penetration of the security windowis the passage of contraband, a narrow slit extending between the busbars may be sufficient for the unlawful purpose without causingasufficient change in resistance to trigger the alarmThis possibility iseffectively forestalled with a security window of the type illustratedin FIG. 7. t

As illustrated inthis embodimentat least the central portion of thesecurity window has two spaced apart conductive'layersextending overmost of the-area of the window. A first pair of bus bars 72 are providedalong the opposite side edges of the security window in electricalcontact with the edges of one of the conductive'layers. Orthogonal tothis first set of bus bars is a second pair of bus bars 73 in electricalcontact with the edges of the second conductive layer within the window.The isolation lines in the conductive layers between the opposed busbars have been deleted from FIG. 7 for enhancing clarityof the drawing.It will be readily apparent that no penetration of the window can bemade that-does not have a component parallel to one or the other of thetwo pairs of bus bars. lt is therefore substantially 'impossibletopenetrate such a window with anyreasonable size hole without triggeringan alarm.

dow initially flat sheets of polyvinyl butyral for the interlayers areassembled in a sandwich and during the heat and pressure cycle oflamination this relatively soft material deforms so that the respectivebus bar imbeds therein. The effect of plural layers for detectingpenetration can also be obtained by depositing thin metal films on bothfaces of the carrier film and laminating that carrier in a window withbus bars in contact with both metal layers.

FIG. 9 illustrates in fragmentary cross section another embodiment ofsecurity window suitable for use in locations where customary access isalmost entirely on one side of the window. This laminated structure hasa glass ply 81 on one face, such as, for example, one quarter inchtempered glass. A polyvinyl butyral interlayer 82 is bonded to theglass. A carrier film 83 having a thin conductive metal layer (notshown) thereon is bonded to the plastic interlayer 82. A secondinterlayer 84 bonds the carrier film 83 to a relatively thick ply oftransparent polycarbonate plastic 86. The exposed face of the plasticply 86 is coated with a protective layer 87 such as chemically depositedsilica, titania or the like,

I which affords a substantial degree of abrasion resis- The arrangementof bus bars-in the window illustrated in FIG. 7 is as simple as possibleand, if desired,

arrangements such as illustrated in FIGS. 4 and 6 may abridge. There isa possibility, although remote, that penetration of both layers couldcause compensating resistance changes in the two'layers when they areused as adjacent arms of .a bridge. The'two layers can be used as,opposite arms of the bridge so that penetration of both layers causes anincrease in sensitivity.

FIG. 8 illustrates in fragmentary cross section a laminate securitywindow having two conductive layers therein; In-this illustrationsuccessive layers are cut away to best shown the location of the busbars. In this particular example the arrangement of successive layers issymmetrical from the center of the laminate, however, it will beapparent that asymmetrical arrangements are also suitable.

Each face of the laminated security window comprises a glass ply 75. Aplastic interlayer 76 about 0.03

inch thick is bonded to each of the glass plys 75. A carrier film 77 ofpolyethylene terephthalate about 0.005 inch thick and having a thinconductive metal coating 78 thereon is bonded to each of the interlayers76. Centrally located in the laminate is a third interlayer 79 whichbonds the two carrier films together. One of the bus bars 72 is imbeddedin one of-the plastic interlayers 76 so as to be in electrical contactwith one of the conductive layers 78. The bus bar 72 is illustratedschematically rather than showthe corrugations of the preferred bus barhereinabove mentionedpThe other bus bar 73 is imbedded in the oppositeinterlayer 76 so as to be in electrical contact with the otherconductive layer 78. During fabrication of such a laminated wintance andprotection against chemical attack on plastic.

Such an asymmetrical laminated security window may be used, for example,in an institution wherein the glass layer 81 is used on the inside wherethe inhabitants have access to the window. The plastic layer would beused on the outside where there is no regular day-today contact.Similarly in a store window or the like the glass layer may be employedon the outside with the plastic layer 87 on the inside where only storepersonnel may have access to it. This is desirable since the plastic plyis softer than the glass and can be scratched.

FIG. 10 illustrates a security window such as might be used fortemporary purposes. In this embodiment a pair of carrier films 89 havingthin conductive metal layers 90 thereon are bonded together with aplastic layer 91 which may be a polyvinyl butyral interlayer as herein-vabove described or may be other suitable adhesive bonding. Therelatively thick interlayer is not generally needed in such a situationsince its ability to conform to the rigid glass and polycarbonate plysof the other embodiments is not a requirement. Care must be taken, ofcourse, to insulate the two metal layers 90 from each other if both areused in active alarm circuits. Similarly the bus bars (not shown) makingcontact to the conductive layers 90 must be insulated. It will also beapparent that if desired a carrier film having a metal coating thereoncan be adhesively bonded to a similar film which serves to protect thedelicate metal layer from damage and a security window suitable fortemporary use may be very inexpensively provided. Bus bars are needed tomake contact with the metal layer. Such a security window made with thinplastic films has considerable flexibility and is light in weight makingit quite suitable for temporary use. Such a lamination of plastic filmswith a conductive layer therein can be bonded on a window and connectedto suitable detection and alarm circuits for forming a security window.Preferably the conductive layer in such a security window is scribedwith electrical isolation lines and electrically connected in a bridgecircuit in one of the manners hereinabove described.

. t I13 ,Thereis a distinctadvantagein the arrangement illustrated inFIG. 2 wherein the conductive layer 26 is separated from the temperedglass ply 23 by at least the interlayer 24 and, if desired,'the carrierfilm 25. The plastic isolates the conductive film from the glass layerso that if the glass is merely broken'most, if not all, of theconductive'layer remains-intact. It is a characteristic of temperedglass that'a break propagatesover the entire extent of the glassbreaking it into a very large number of small fragments. If theconductive layer were on the glass or closely coupled thereto suchbreakage of the glass-would completelyruptu're the delicate metal layerand it would appear that penetration was being attempted. With the metallayer decoupled from the glass by a relatively soft resilientintervening plastic layer mere breakage of the glass does not disruptthe conductive film to more than aminor extent. Even if an alarm isticof the response to severance of the film. Thus, for

example,penetration of the security window by a high speed projectilewill cause a large change in resistance as penetration occurs which canbe detected by the AC amplification system. Thus the penetration can bede-- tected bythe rapid pulse of resistance change, even might besounded when theternpered glass is broken the alarmlsystem can be resetto indicate when anat-' teniptis made to penetrate the window.

The resi "ance of theth'in metal film embedded inthelaminatedisecuritywindow is also-sensitive to strain. That is, as thefilm is strained, the resistancechanges. Thus, for example, when theconductive layer is located offfof a neutral axis of the cross sectionof the laminated window, a bending of the window will induce strain inthe conductive layer and changeits' resistance. This discovery gives onean opportunity to employ the deformation of the window prior to.penetration for providing a'n'alarm signal; More particulary atransientchange inresistnacecan be detected with a pulse or rateof, changegreater than some predetermined mag-- nitude. p FIG. 11 illustratesinblockdiagram a system for ulitizing the strain sensitive properties ofthe conductive alarm. A'power supply 93 appli'esipower to the conductivemetal layer in a security window 94. The excitation applied to thewindow by the power supply may be AC or DC, and may be either current orvoltage as desired. The window 94 is also connected to an AC amplifier96 with gain control 97; The AC amplifier may also have a band widthcontrol, if desired, for limiting the AC range amplified; The output ofthe AC amplifieris applied to a threshold detector 98 which applies anoutof-limits signal toan alarm 99.

though the steady state resistance may not be significantlydifierentfromtheoriginal resistance. a t

It may be desirable to employ a'str ain sensitive detection-system assetforth in FIG. 11 with a penetration I sensing system as illustrated inFIG. 3. ln such an arrangement the input to the AC amplifier 96j'may beeither'the output of the null balance bridge of FIG. 3 or the output ofa DC amplifier 41 which may reduce gain requirements of the,AC-amplifier 96.

'If desired, a continuous-surveillance 'monitor lfll may be connected tothe output of the AC amplifier 96.

film in alaminated window for providing a security This continuoussurveillance monitor may have a.visu al or aural output so that anattendant can perceive signal changes, such as, for example, due tosomeone'pounding on a security window. The continuoussurveillancemonitor can also be some means for recording the output signal forreview at a later time. 4

' A resistance bridge is of course not theonly way of detecting a'changein the resista nce of the conductive I layer. A simple and inexpensivetechnique-is illustrated in FIG. 12. As illustrated in this arrangementa voltage e, is applied to a resistor 102 connected to an input of anoperational amplifier 103. A second resistor 104 is connected across theamplifier. The output .voltage 2 is proportional to the input voltageand the ratio of the resistance of resistor 104 to the resistance ofresistor If someone should commence striking or otherwise deforming thewindow in order to effect penetration, the resultant time varying signalis, amplified. When the signal is within the frequency band of theACamplifier and-beyond the preset threshold, an alarm will sound.

Such a system is also responsive to the rate of change of penetrationthrough a window as represented by the changing' lfesistance. In animpact sensitive system one can use the'magnitude of the pulse ofchanging resistance to detect a penetration or 'attemptfat penetration.

If desired the rate of change or rise time of the pulse or pulsewidthcan beselected for triggering an alarm. The circuitry fordetecting anysuch characteristic of chang ing resi'stanceis conventional. I

The sensitivity of the threshold detector 98 can be set so that thestrain required to activiate the alarm 99 is a large fraction 'of thestrain that would occur in the window .before'breakage. Gaincontrol 97may effect this function. Thus, relativelyminor blowsona window whichare far short of causing breakage can be ignored and apulse representing'asufficien't'blow to be quite 102. The output voltage-is thus'quitesensitive to any change in the relative valuesof the twol'resistors-Aconductive layer in a security window can be used for either of the tworesistors in this schematic diagram, or if desired two conductive areasin a window could be used as both resistors 102 and 104 for temperaturecompensation. Theoperational amplifier can be either a single inputamplifier, or can be a differentialamplifier with two inputs.

One can also use ohmmeter circuits or a variety of current or voltagecomparison circuits for noting a change in resistance due to windowpenetration. It will also be noted that the resistance values can bedigitized at any. point in the electrical .circuit' and digitaltechniques used fol-balancing, comparing andthe like. ,If the resistanceof a conductive layeris digitized it can becompared to adigitalreference number. and the balance can be maintained by changingthe referen'ce number. As much or as little sophistication as desiredcan be achieved with digital techniques for comparison, correction andavoidance of false alarms or tampering with the system. Many otherarrangements will be apparentto one skilled in the art for detectingavariation in resistance of the security window.

Although limited embodiments of security window and alarmsystemsassociated therewith have been described and illustrated herein,many modifications and variations will-be apparent to one skilled in theart. It is, therefore, to be understood that within the scope of theappended claims the invention may be practiced otherwise than as'specifically described.

What is claimed is:

1. A security window system comprising: a transparent electricallyconductive layer extending over most of the areaof the window;

a set of bus bars in electrical contact with opposite edge portions ofthe conductive layer; means connectedto the bus bars for detecting ananalog change in resistance of the conductive layer; and I means forproviding an output signal in response to a detected change inresistance in excess of a predetermined magnitude.

2. A security window system as defined in claim 1 further comprisingmeans for resetting the system after a change in resistance forprovidingan output signal in response to a subsequent analog change inresistance.

3. A security window system comprising:

a transparentelectrically conductive layer extending over mostof thearea of the window;

a set'of bus bars in electrical contact with opposite edge portions oftheconductive layer;

means connected to the bus bars for detecting an analog change inresistance of the conductive layer;

means for providing an output signal in response to a detected change inresistance in excess of a predetermined magnitude;

means for resetting the system after a change in resistance forproviding an output signal in response to a subsequent analog change inresistance; and

' means for recording cumulative changes in resistance.

4. A security window system comprising:

a transparent electrically conductive layer extending over most of thearea of the window and divided into at least two conductive areas;

a set of bus bars in electrical contact with opposite edge portions ofthe conductive layer;

means connected to the bus bars for detecting an analog change inresistance of the conductive layer comprising means for detecting achange in'the relative resistances of the two areas; and

means for providing an output signal in response to a detected change inresistance in excess of a predetermined magnitude.

5. A security window system as defined in claim 4 wherein-the means fordetecting comprises a bridge and the two conductive areas are inadjacent arms of the bridge.

6'. A security window system as defined in claim 1 wherein the means fordetecting comprises a bridge and wherein the conductive layer is in onearm of the bridge. n r

7. A security window system comprising:

a transparent electrically conductive layer extending over most of thearea of the window;

a set of bus bars in electrical contact with opposite edge portions ofthe conductive layer;

means connected to the bus bars for detecting an analog change inresistance of the conductive layer comprising a resistance bridge; and

means for providing an output signal in response to a detected change inresistance in excess of a predetermined magnitude; and wherein theconductive layer is divided into at least four conductive areas each inan arm of the bridge.

8. A security window system as defined in claim 1 comprising:

a frangible layer; and

a resilient layer isolating the electrically conductive layer from thefrangible layer.

it. A security window system as defined in claim 1 wherein theelectrically conductive layer is laminated between a pair of rigid faceplies and including a resilient layer between the conductive layer andeach of the rigid plies.

10. A security window system comprising:

a'transparent sheet; an electrically conductive layer extending overmost of the area of the transparent sheet for making an analog variationin an electrical signal applied thereto in response to penetration ofthe transparent sheet; means for applying an electrical signal to theconductive layer; and

means connectedto the conductive layer for detecting a variation inelectrical signal greater than a predetermined limit.

11. A security window system as defined in claim 10 further comprisingmeans for resetting the system after a change in resistance fordetecting a subsequent change in resistance greater than a predeterminedlimit.

12. A security window system comprising:

a transparent sheet;

an electrically conductive layer laminated within the transparent sheetand extending over most of the area of the transparent sheet for makingan analog variation in an electrical signal applied thereto in responseto penetration of the transparent sheet;

means for applying an electrical signal to the conductive layer; and

means connected to the conductive layer for detect ing a variation inelectrical signal greater than a predetermined limit; and wherein theconductive layer is divided into at least two conductive areas and themeans for detecting is connected thereto for detecting a change in therelative resistances of the two areas.

13. A security window system'comprising:

a transparent sheet;

an electrically conductive layer laminated within the transparent sheetand extending over most of the area of the transparent sheet for makingan analog variation in an electrical signal applied thereto in responseto penetration of the transparent sheet;

means for applying an electrical signal to the conductive layer; and

means connected to the conductive layer for detecting a variation inelectrical signal greater than a predetermined limit; and wherein theconductive layer is divided into at least two conductive areas inadjacent areas in adjacent arms of a resistance bridge.

14. A security window system comprising:

va transparent sheet;

an electrically conductive layer laminated within the transparent sheetand extending over most of the 7 area of the transparent sheet formaking an analog variation in an electrical signal applied theretoin vresponse to penetration of the transparent sheet;

' means for applying an electrical signal to the conductive layer; and ameans connected to the conductive layer for detecting a variation inelectrical signal greater than a predetermined limit; and wherein theconductive layer is divided into at least four conductive areas each inan arm of a resistance bridge.

15. A laminated security window system comprising:

an electrically conductive layer within the laminated window andextending over most of the area of the window; 7 means for makingelectrical contact with edge portions of the conductive layer forapplying an elecsecond value to a third value.

1. A security window system comprising: a transparent electricallyconductive layer extending over most of the area of the window; a set ofbus bars in electrical contact with opposite edge portions of theconductive layer; means connected to the bus bars for detecting ananalog change in resistance of the conductive layer; and means forproviding an output signal in response to a detected change inresistance in excess of a predetermined magnitude.
 2. A security windowsystem as defined in claim 1 further comprising means for resetting thesystem after a change in resistance for providing an output signal inresponse to a subsequent analog change in resistance.
 3. A securitywindow system comprising: a transparent electrically conductive layerextending over most of the area of the window; a set of bus bars inelectrical contact with opposite edge portions of the conductive layer;means connected to the bus bars for detecting an analog change inresistance of the conductive layer; means for providing an output signalin response to a detected change in resistance in excess of apredetermined magnitude; means for resetting the system after a changein resistance for providing an output signal in response to a subsequentanalog change in resistance; and means for recording cumulative changesin resistance.
 4. A security window system comprising: a transparentelectrically conductive layer extending over most of the area of thewindow and divided into at least two conductive areas; a set of bus barsin electrical contact with opposite edge portions of the conductivelayer; means connected to the bus bars for detecting an analog change inresistance of the conductive layer comprising means for detecting achange in the relative resistances of the two areas; and means forproviding an output signal in response to a detected change inresistance in excess of a predetermined magnitude.
 5. A security windowsystem as defined in claim 4 wherein the means for detecting comprises abridge and the two conductive areas are in adjacent arms of the bridge.6. A security window system as defined in claim 1 wherein the means fordetecting comprises a bridge and wherein the conductive layer is in onearm of the bridge.
 7. A security window system comprising: a transparentelectrically conductive layer extending over most of the area of thewindow; a set of bus bars in electrical contact with opposite edgeportions of the conductive layer; means connected to the bus bars fordetecting an analog change in resistance of the conductive layercomprising a resistance bridge; and means for providing an output signalin response to a detected change in resistance in excess of apredetermined magnitude; and wherein the conductive layer is dividedinto at least four conductive areas each in an arm of the bridge.
 8. Asecurity window system as defined in claim 1 comprising: a frangiblelayer; and a resilient layer isolating the electrically conductive layerfrom the frangible layer.
 9. A security window system as defined inclaim 1 wherein the electrically conductive layer is laminated between apair of rigid face plies and including a resilient layer between theconductive layer and each of the rigid plies.
 10. A security windowsystem comprising: a transparent sheet; an electrically conductive layerextending over most of the area of the transparent sheet for making ananalog variation in an electrical signal applied thereto in response topenetration of the transparent sheet; means for applying an electricalsignal to the conductive layer; and means connected to the conductivelayer for detecting a variation in electrical signal greater than apredetermined limit.
 11. A security window system as defined in claim 10further comprising means for resetting the system after a change inresistance for detecting a subsequent change in resistance greater thana predetermined limit.
 12. A security window system comprising: atransparent sheet; an electrically conductive layer laminated within thetransparent sheet and extending over most of the area of the transparentsheet for making an analog variation in an electrical signal appliedthereto in response to penetration of the transparent sheet; means forapplying an electrical signal to the conductive layer; and meansconnected to the conductive layer for detecting a variation inelectrical signal greater than a predetermined limit; and wherein theconductive layer is divided into at least two conductive areas and themeans for detecting is connected thereto for detecting a change in therelative resistances of the two areas.
 13. A security window systemcomprising: a transparent sheet; an electrically conductive layerlaminated within the transparent sheet and extending over most of thearea of the transparent sheet for making an analog variatiOn in anelectrical signal applied thereto in response to penetration of thetransparent sheet; means for applying an electrical signal to theconductive layer; and means connected to the conductive layer fordetecting a variation in electrical signal greater than a predeterminedlimit; and wherein the conductive layer is divided into at least twoconductive areas in adjacent areas in adjacent arms of a resistancebridge.
 14. A security window system comprising: a transparent sheet; anelectrically conductive layer laminated within the transparent sheet andextending over most of the area of the transparent sheet for making ananalog variation in an electrical signal applied thereto in response topenetration of the transparent sheet; means for applying an electricalsignal to the conductive layer; and means connected to the conductivelayer for detecting a variation in electrical signal greater than apredetermined limit; and wherein the conductive layer is divided into atleast four conductive areas each in an arm of a resistance bridge.
 15. Alaminated security window system comprising: an electrically conductivelayer within the laminated window and extending over most of the area ofthe window; means for making electrical contact with edge portions ofthe conductive layer for applying an electrical signal thereto; meansfor detecting a change in resistance of the conductive layer from afirst value to a second value; and means for providing an output signalwhen the change in resistance from the first value to the second valueis in excess of a predetermined magnitude.
 16. A security window systemas defined in claim 15 further comprising means for resetting the systemafter a change in resistance for providing an output signal in responseto a subsequent change of resistance from the second value to a thirdvalue.